*2.1.5. Use of expired vaccines*

The potency of vaccines is maintained to a certain period of time, provided that the transportation and storage temperature is properly maintained. The use of vaccines after the date of expiry may not result in optimal immune response and can also result in vaccine failure.

#### *2.1.6. Mutation of viruses*

Some of the viruses like the influenza virus are of a mutating nature and as a result pose a serious threat regarding the effectiveness of vaccine against certain diseases.

#### **2.2. Host factors**

The poultry birds to be vaccinated against diseases may not respond effectively against vaccines due to the following shortcomings, thus resulting in vaccine failure.
